Wastewater irrigation hazard or lifeline

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Wastewater irrigation - hazard or lifeline?
Gez Cornish Neeltje Kielen HR Wallingford Ltd.
UK
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Wastewater irrigation - hazard or lifeline?
Summary

• Based on field studies in Nairobi and Kumasi
• The need for a typology of wastewater irrigation
• Variations in microbiological water quality
• Positive impacts of wastewater irrigation
• What are the trade-offs?
• The need for interim water quality standards

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Wastewater irrigation - hazard or lifeline?
The need for a typology of wastewater irrigation

• Great variation exists in wastewater
• sources
• conveyance systems
• treatment
• in-field management
• A typology is essential to guide discussion of
  practice or the formulation of guidelines or
  regulations.

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Wastewater irrigation - hazard or lifeline?
A proposed typology
Conveyance
Treatment
Disposal
Source
Formal collection network
River or surface water body
Conventional
Industrial
Road tankers
Natural / Biological
Groundwater recharge
Municipal
Natural drainage
None
Irrigation
Informal Backyard
Informal or formal use
Indirect use
Direct use
Indirect use
Direct use
Informal use
Formal use Use of wastewater with a certain
level of permission and control by state
agencies Informal use Use of wastewater lacking
permission and control by state
agencies Direct use Wastewater conveyed to
a defined area for irrigation Indirect use
Wastewater discharged into water bodies with
scattered and uncontrolled downstream abstraction
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Wastewater irrigation - hazard or lifeline?
Examples - Mau Mau Bridge, Nairobi
Informal, indirect. Untreated municipal waste.
Natural drainage channels running to river.
Surface overhead irrigation of vegetable crops
sold in local Nairobi markets.
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Wastewater irrigation - hazard or lifeline?
Examples - Maili Saba, Nairobi
Informal, indirect. Untreated municipal waste
diverted from sewer to fields. Surface
irrigation of vegetable crops grown for home
consumption and sold in local Nairobi markets.
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Wastewater irrigation - hazard or lifeline?
Examples - Asago, Kumasi
Informal, indirect. Untreated municipal
industrial waste. Natural drainage to river
dumping by vault emptying tankers. Overhead
irrigation of vegetable crops, grown mainly for
Kumasi market.
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Wastewater irrigation - hazard or lifeline?
Variations in water quality - Nairobi
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Wastewater irrigation - hazard or lifeline?
Variations in water quality - Kumasi
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Wastewater irrigation - hazard or lifeline?
Positive impacts
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Wastewater irrigation - hazard or lifeline?
Positive impacts
Nairobi
Kumasi
Kale, tomato, spinach, green maize, cabbage
Tomato, garden egg, okra, chilli
Main crops
Av. Revenue US / ha
1,770 (annual)
544 (7 months)
Total value of production
US 3.9 million
US 6 million
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Wastewater irrigation - hazard or lifeline?
Positive impacts - revenue profit, Nairobi
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Wastewater irrigation - hazard or lifeline?
Positive impacts - profits, Kumasi
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Wastewater irrigation - hazard or lifeline?
What are the trade-offs
Costs
Benefits

• HH income
• Urban food supply
• Nutrient recycling
• Managed waste disposal

• Chronic ill-health
• Wide-scale disease outbreaks
• Damage to soils groundwater

Where is the greatest public good secured?
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Wastewater irrigation - hazard or lifeline?
Interim water quality standards

• Current WHO guidelines
• Apply to TREATED wastewater
• Aim to secure no measurable excess risk of
  infection

• Use of UNTREATED wastewater is widespread. Under
  these conditions
• No risk standard seems unrealistic
• A single threshold is unhelpful - all wastewater
  is not equal
• Are there acceptable levels of risk what do
  they imply?
• Can we provide greater guidance - risk
  assessment scales?
• Can quantitative microbiological risk assessment
  models provide these answers?